1. Field of the Invention
The present invention relates to fuel cells and their operation.
2. Description of the Related Art
A fuel cell generates electric power through reaction of a fuel gas fed to a fuel electrode and an oxygen-containing gas fed to an oxygen electrode. As the fuel gas, hydrogen supplied from a hydrogen cylinder or a reformed gas obtained by reforming a city gas to enrich the hydrogen content are used. As the oxygen-containing gas, air is generally fed with a compressor or a blower. An electrode of a fuel cell is generally made of an electroconductive carbon having a noble metal carried on the surface thereof. In a fuel cell using a polyelectrolyte, a fuel gas containing hydrogen electrochemically reacts with an oxidizing agent gas such as oxygen-containing air, thereby simultaneously generating an electric power and heat.
A catalyst used on the electrode of the fuel cell is gradually oxidized on the surface thereof upon being exposed to an oxidative atmosphere, and adsorbs contaminants in the air and contaminants leaked from the apparatus on the surface of the catalyst. The reaction efficiency of the catalyst is lowered thereby, and thus the generated voltage is lowered with the lapse of time. In order to solve the problem, it has been proposed that in the shutdown period of the fuel cell, an inert gas, such as a nitrogen gas, is charged to prevent oxidation of the electrodes, and the fed gas is fed through a filter to decrease the amount of contaminants in the gas. However, these measures cannot restore the voltage having been once lowered although the lowering of the generated voltage can be suppressed to prolong the service life. Furthermore, a fuel cell has such a nature that the generated voltage thereof is eventually lowered despite of the effect of prolonging the service life.
In the case where the gas is fed through a filter, it is necessary to exchange the filter on a regular schedule to cause such a problem of consuming labor and cost for exchanging the filter. Furthermore, additional energy is necessary in the compressor or the blower corresponding to the pressure loss of the filter.
In the case where the cathode is at a high potential, due to the event that the fuel cell holds a very high voltage exceeding 0.9 V, which is close to the open circuit state, it is already understood that problems such as elution of the Pt catalyst of the cathode and reduction in reaction area of the Pt catalyst due to sintering (enlargement of Pt particles) occur.
Similarly, in the case where the fuel cell holds a very high voltage, which is close to the open circuit state, there occurs a problem that the polyelectrolyte decomposes. It is considered that such problems are caused by the following reasons.
An open circuit voltage of the fuel cell utilizing hydrogen and oxygen as reaction seeds is theoretically considered to be 1.23 V. However, the actual open circuit voltage is a mixed potential of impurities in the respective electrodes, i.e., an anode and a cathode, and adsorption seeds and is from about 0.93 V to 1.1 V. Also, the open circuit voltage is lowered from the theoretical value due to the event that hydrogen and oxygen are slightly diffused in the polyelectrolyte membrane. Assuming that no dissolution of impurities such as radical metal seeds occurs, the potential of the anode is greatly influenced by the adsorption seeds of the cathode and is considered to become a mixed potential of chemical reactions expressed by the following reaction equations 1 to 5 as described in H. Wroblowa, et al., J. Electroanal. Chem., 15, pp. 139-150 (1967), “Adsorption and Kinetics at Platinum Electrodes in the Presence of Oxygen at Zero Net Current”. Incidentally, the voltage expressed as corresponding to each of the reaction equations shows the standard electrode potential against a standard hydrogen electrode when the reaction expressed by the subject reaction equation occurs. In the case where the potential of the anode is high in this way, it is considered that hydroxide radical (OH.), super oxide (O2−.), and hydrogen radical (H.) are generated in high concentrations, whereby these radicals attack a part having high reactivity in the polyelectrolyte to decompose the polyelectrolyte.
Reaction equation 11.23 VO2 + 4H+ + 4e− = 2H2OReaction equation 21.11 VPtO2 + 2H+ + 2e− = Pt(OH)2Reaction equation 30.98 VPt(OH)2 + 2H+ + 2e− = Pt + 2H2OReaction equation 40.88 VPtO + 2H+ + 2e− = Pt + H2OReaction equation 50.68 VO2 + 2H+ + 2e− = H2O2
In order to avoid the foregoing problems caused by the event that the fuel cell becomes in the state of open circuit voltage, there have hitherto been proposed some operation methods of fuel cell system.
For example, there is proposed an operation method of a fuel cell system in which an electric power consumption measure for consuming an electric power is provided within the fuel cell system individually from an external load, and the fuel cell and the electric power consumption measure are connected during a period of time until the fuel cell and the external load are connected after the fuel cell starts power generation, whereby the electric power formed in the fuel cell is consumed by the electric power consumption measure, thereby avoiding that the fuel cell approaches the state of open circuit voltage (for example, see JP-A-5-251101).
Also, there is proposed an operation method of a fuel cell system in which a discharge measure for suppressing an open circuit voltage is provided within the fuel cell system, thereby avoiding the event that the fuel cell becomes in the state of open circuit voltage (for example, see JP-A-8-222258).
According to these operation methods of fuel cell systems, it is possible to avoid the foregoing elution of the Pt catalyst of the cathode and reduction in reaction area of the catalyst due to sintering. Also, it is possible to avoid the event that the polyelectrolyte is decomposed due to the formation of radicals.
However, in the case of the foregoing operation method of a fuel cell system by purging with an inert gas such as nitrogen, there arises a problem that not only a gas cylinder of the inert gas is necessary, leading to enlargement in size of the fuel cell system, but also the maintenance such as exchange of the gas cylinder is expensive, leading to an increase in costs.
Also, in the foregoing operation method of a fuel cell system by purging with water or a moistened inert gas, since the temperature of the fuel cell is lowered at the time of stopping the power generation of the fuel cell, dew condensation occurs inside the fuel cell, and the volume is reduced. Accordingly, since the inside of the fuel cell becomes in the state of negative pressure, there arises problems that oxygen enters from the outside and that the polyelectrolyte membrane breaks, leading to potential occurrence of a short circuit of the electrodes.
Also, in the case where the cell is subjected to power generation in the state of stopping feeding of the oxidizing agent gas to consume oxygen of the cathode, and the anode is then purged with an inert gas, there is a problem in that since the Pt catalyst of the cathode is oxidized with oxygen remaining without being consumed and air incorporated due to diffusion and leakage, the cathode is degraded. Moreover, there arises a problem that since oxygen is forcedly consumed by power generation, the potential of the cathode is not uniform, and the activation state of the cathode varies every time when the power generation of the fuel cell is stopped, leading to scattering of the cell voltage at the time of start.
In addition, in the case of the foregoing operation method of a fuel cell system by avoiding the matter that the fuel cell becomes in the state of open circuit voltage, the fuel cell is always in the state of power generation. However, in the case of a fuel cell system for household use using a raw material gas such as town gas containing methane as the major component, in order to suppress the heating and lighting expenses, it is desired to control the motions of the fuel cell so as to stop the power generation in a time period where the electric consumption is small and carry out the power generation in a time period where the electric consumption is large. For example, according to the DSS (Daily Start-up & Shut-down) wherein the power generation is carried out in the daytime and is stopped in the middle of the night, it is possible to avoid an increase in the heating and lighting expenses. Therefore, it is desired to control the fuel cell so as to repeat the power generation state and the non-power generation state, and an operation method of a fuel cell system capable of avoiding the matter that the fuel cell approaches open circuit voltage even when repeat the power generation state and the non-power generation state, is desirable.
All references, patents and priority documents, particularly Japanese Patent Application 2002-317794 filed Oct. 31, 2002, referred to herein, are hereby incorporated by reference for the entirety of their disclosure for all purposes.